The background description provided herein is for the purpose of generally presenting the context of the present invention. The subject matter discussed in the background of the invention section should not be assumed to be prior art merely as a result of its mention in the background of the invention section. Similarly, a problem mentioned in the background of the invention section or associated with the subject matter of the background of the invention section should not be assumed to have been previously recognized in the prior art. The subject matter in the background of the invention section merely represents different approaches, which in and of themselves may also be inventions.
Currently, in the optical fiber manufacturing, there are mainly two types of novel single-mode optical fiber products which are quite popular. One is a G652 optical fiber with ultra low attenuation. Because of a low attenuation coefficient and good compatibility performance, the G652 optical fiber with ultra low attenuation becomes one of representatives of future novel optical fibers. The other one is a G654 optical fiber with a large effective area. By increasing an effective area of the optical fiber and restraining the non-linear effect during optical fiber transmission, the G654 optical fiber with the large effective area is more suitable for a long-distance large-capacity transmission system.
For optical fiber manufacturing industries, it is a great challenge to find an effective method to reduce an optical fiber attenuation coefficient of both the G652 optical fiber with ultra low attenuation and the G654 optical fiber with a large effective area, and control manufacturing costs. The main difficulties lie in the following three aspects:
1. How to decrease attenuation: Currently, a main method is to decrease a Rayleigh scattering coefficient of an optical fiber by controlling the composition of a glass material and controlling a thermodynamics changing process of the glass in a manufacturing process.
2. The optical fiber manufacturing process needs to be simple and controllable, and optical fiber manufacturing costs should not be significantly increased. Currently, controlling of the current ultra low attenuation process is complex. Especially for a design of a currently common pure silicon core layer with ultra low attenuation, to ensure total reflection of the optical fiber, a cladding uses an outer cladding layer material doped with only fluorine. The manufacturing process is complex, and costs of the optical fiber are greatly affected.
3. Besides an ultra low attenuation coefficient, it needs to ensure that optical parameters of the optical fiber satisfy the ITU-T standard, which mainly means that a mode field diameter (MFD), a chromatic dispersion, a cut-off wavelength, and a flexibility property need to be controlled within ranges required by the standard. That is, not only ultra low attenuation performance of the optical fiber needs to be ensured, but also other optical parameters need to be controlled within corresponding ranges.
For the foregoing three difficulties, how to decrease attenuation of the optical fiber is specifically discussed first. Attenuation of a silica optical fiber at a wavelength range from 600 nm to 1600 nm is mainly caused by Rayleigh scattering, and attenuation αR caused by Rayleigh scattering may be calculated according to the following equation:
      α    R    =                    1                  λ          4                    ⁢                        ∫          0                      +            ∞                          ⁢                              R            ⁡                          (              r              )                                ⁢                      P            ⁡                          (              r              )                                ⁢                      rdr            /                                          ∫                0                                  +                  ∞                                            ⁢                                                P                  ⁡                                      (                    r                    )                                                  ⁢                rdr                                                          =                  R                  λ          4                    +      B      where λ is a wavelength (μm); R is a Rayleigh scattering coefficient (dB/km/μm4); P is a light intensity; and when the Rayleigh scattering coefficient is determined, B is a corresponding constant. Therefore, the attenuation αR (dB/km) caused by Rayleigh scattering may be obtained as long as the Rayleigh scattering coefficient R is determined. Rayleigh scattering is caused by density fluctuation in one aspect, and caused by concentration fluctuation in another aspect. Accordingly, the Rayleigh scattering coefficient R may be represented as:R=Rd+Rc where Rd and Rc respectively represent changes in the Rayleigh scattering coefficient caused by the density fluctuation and the concentration fluctuation. Rc is a concentration fluctuation factor, and is mainly affected by a doping concentration of glass of the optical fiber. Theoretically, Rc is smaller when less Ge and F or other dopants are used. Because of this, currently, some foreign enterprises use a design of a pure silicon core layer to achieve ultra low attenuation performance.
It should be noted that the Rayleigh scattering coefficient further includes another parameter Rd. Rd is related to a fictive temperature TF of the glass, and changes as the structure and the temperature of the glass change. The fictive temperature TF of the glass is a physical parameter that represents the structure of the glass, and is defined as a temperature corresponding to a balanced state in which the structure of the glass is no longer adjusted when the glass is rapidly cooled from a temperature T′ to a room temperature. When T′>Tf (a softening temperature of the glass), because the glass has a relatively small viscosity, and the structure of the glass is easy to be adjusted, at each moment, the glass is in the balanced state, and therefore, TF=T′. When T′<Tg (a transition temperature of the glass), because the glass has a relatively large viscosity, and the structure of the glass is difficult to be adjusted, structure adjustment of the glass lags behind the temperature change, and therefore, TF>T′. When Tg<T′<Tf (the softening temperature of the glass), the glass needs a relatively short time to tend to a balance. A specific time is related to the composition and a cooling speed of the glass, and therefore, TF>T′ or TF<T′.
When a design of a pure silicon core layer is used, to ensure total reflection of an optical fiber, the pure silicon core layer needs to match an inner cladding layer doped with fluorine (F) that has a relatively low refractive index, to ensure a sufficient refractive index difference maintained between the core layer and the inner cladding layer. Therefore, the core layer of the pure silicon core layer has a relatively high viscosity, and the inner cladding layer doped with a large amount of F has a relatively low viscosity. As a result, viscosity matching of the optical fiber structure is unbalanced, and the virtual temperature of the optical fiber having the pure silicon core layer structure increases rapidly, causing an increase in Rd of the optical fiber. Consequently, benefits brought by a decrease in Rc are counteracted, and abnormality in attenuation of the optical fiber may further be caused instead.
In view of the foregoing, theoretically, an ultra low attenuation coefficient cannot be obtained simply by reducing dopants in a core layer. To resolve this problem, U.S. Publication No. US2010/0195999A1 discloses a method in which an alkali metal is added to a core layer is used. The core layer of an optical fiber still uses a pure silicon core layer, and an increase in Rd caused by viscosity mismatching is resolved by changing a viscosity of the core layer of the optical fiber and a structural relaxation time of the core layer, so as to overall decrease a Rayleigh scattering coefficient of the optical fiber. However, although optical fiber attenuation according to this method can be effectively decreased, a manufacturing process is relatively complex, a core layer rod needs to be processed in multiple batches, and there is an extremely high requirement on controlling of the doping concentration of the alkali metal, hindering manufacturing of the optical fiber in a large scale.
Chinese Application No. CN201310394404 provides a design of an optical fiber with ultra low attenuation. The optical fiber uses a design of a pure silicon dioxide outer cladding layer. However, because the optical fiber uses a typical step profile structure, and does not use a design of a trench cladding layer to optimize bending of the optical fiber, and a core layer of the optical fiber is not doped with Ge, viscosity mismatching may occur during manufacturing of a preformed rod, and it may be found that the optical fiber has undesirable attenuation and a relatively poor bending level.
U.S. Publication No. US2010022533 discloses a design of an optical fiber as so to obtain a lower Rayleigh coefficient. The optical fiber uses a design of a pure silicon core layer, germanium and fluorine are not doped in the core layer, and silicon dioxide doped with fluorine is used as an outer cladding layer in the design of the optical fiber. Such a design of the pure silicon core layer requires complex viscosity matching inside the optical fiber, and an extremely low speed needs to be used in a drawing process, to avoid an increase in attenuation resulted from defects inside the optical fiber caused by high-speed drawing. The manufacturing process is extremely complex.
It can be found from the foregoing description that, to obtain a reduced attenuation coefficient, if the design of a pure silicon core layer, or the design of a core layer not doped with germanium (Ge) is used, the composition of a core layer material needs to be strictly controlled, so that the viscosity of the core layer material matches that of an outer cladding layer material, and an increase in Rd of the optical fiber is reduced.
However, as is well known, from the point of view of process implementations, it is quite complex to control the composition of a core layer material, especially when an alkali metal or another element that reduces a virtual temperature of an optical fiber is added to a core layer. Consequently, manufacturing costs of the optical fiber are increased. In addition to doping an alkali metal in the core layer of the pure silicon core layer, can the same effect be achieved by designing the viscosities of the outer cladding layer and the inner cladding layer? As is known, the virtual temperature of the core layer of the optical fiber is affected by the composition of the outer cladding layer material. Therefore, by designing the viscosities of the outer cladding layer and the inner cladding layer, and especially by doping metal ions to an outermost layer, that is, the outer cladding layer, of the glass of the optical fiber, a material relaxation time of each part of the optical fiber material can be significantly changed, thereby changing the virtual temperature of the optical fiber. Therefore, the concept of a non-pure silicon core layer can be used in the process, and by suitably designing the viscosities of the parts of the optical fiber, a simple manufacturing method for the core layer may be found, thereby achieving an optical fiber with ultra low attenuation.
The second difficulty in achieving the optical fiber with ultra low attenuation is cost control. For a common design of a single-mode optical fiber with ultra low attenuation, a design of an outer cladding layer doped with only fluorine is used. From the point of view of optical fiber optics, such a design is relatively simple, and a requirement on total reflection of the optical fiber can be satisfied, as long as a refractive index difference between the outer cladding layer and the core layer is ensured. However, currently, there are three main factors that restrain manufacturing costs of the optical fiber with ultra low attenuation. First, currently, manufacturing costs of mainstream alkali metal processes are high, and efficiency is low. Second, a preformed rod using the design of doping only fluorine has a relatively small size, and a drawing process is complex. Third, because an F doping process is used, manufacturing costs of the optical fiber using the design of doping only fluorine are quite high. According to preliminary estimation by using current market prices, the price of an F-doped tube is five to eight times the price of a pure silicon dioxide tube. According to calculation by using the initial relationship that costs of an F-doped material is six times costs of a pure silicon dioxide material, if the thickness of an F-doped layer is appropriately reduced by using an appropriate process design, manufacturing costs of the optical fiber can be significantly reduced. Assuming that an F-doped material is used only at positions of the diameter of the optical fiber from 30 microns to 80 microns, and a common pure silicon dioxide is used at positions from 80 microns to 125 microns, material costs of this design is reduced by 40% compared with a conventional design of an optical fiber with ultra low attenuation that only uses the F-doped material. If the F-doped material is used at positions from 30 microns to 60 microns, and the common pure silicon dioxide is used at positions from 60 microns to 125 microns, material costs of this design is reduced by 65%.
It can be found from the foregoing analysis that, a process design of an optical fiber with ultra low attenuation that uses both a non-pure silicon core layer and a cladding doped with fluorine is feasible. Affected by the foregoing two restrictive factors, the final challenge confronted is to control optical parameters of the optical fiber in such a design.
Because if pure silicon dioxide not doped with fluorine is used as the outer cladding layer material, there are three problems.
First, a cutoff of a fundamental mode is restrained: A relatively small refractive index difference between the outer cladding layer material and the core layer material causes fundamental mode leakage of the optical fiber, and further affects attenuation of the optical fiber. If an optical fiber with ultra low attenuation that is designed with the outer cladding layer material not doped with F is used, an optical fiber section needs be appropriately designed at the middle position between the outer cladding layer and the core layer, to restrain the fundamental mode leakage.
Second, viscosity matching is considered: If no viscosity optimization design is made for the outer cladding layer material, and the viscosity of the outer cladding layer material does not match a viscosity gradient between the inner cladding layer and the core layer, problems such as defects of boundary surface positions and an increase in the virtual temperature are caused, and consequently, optical fiber attenuation increases.
Third, matching of optical sections is considered: If pure silicon dioxide glass is used as the outer cladding layer material, when a viscosity matching design is considered, the doping concentration of each part is confined. To ensure that the optical parameter of the optical fiber satisfies the parameter requirements of the G652 or G654 optical fiber, that is, to ensure that the MFD, the chromatic dispersion, and the bending performance of the optical fiber can satisfy the standard requirements, an optical section design needs to be considered. Therefore, it is required that when the viscosity is designed, an optical design of the optical fiber should be taken into overall consideration. This increases a difficulty in process implementation.
Therefore, a heretofore unaddressed need exists in the art to address the aforementioned deficiencies and inadequacies.